We have isolated actin from Acanthamoeba castellanii, human blood platelets, embryonic chick brain, and rat liver with particular attention to identifying any species of actin whose polymerization is qualitatively different from that of rabbit skeletal muscle actin. No such species of actin was found. The failure of non-muscle actin to polymerize in crude extracts of these cells must then be attributed to the presence of regulating factors. The identification of such factors and their possible role in cell motility is the subject of future investigation. BIBLIOGRAPHIC REFERENCES: Gordon, D.J., Eisenberg, E., Korn, E.D.: Characterization of cytoplasmic actin isolated from Acanthamoeba castellanii by a new method. J. Biol. Chem. 241: 4778, 1976. Gordon, D.J., Yang, Y.-Z., Korn, E.D.: Polymerization of Acanthamoeba actin: Kinetics, thermoydynamics, and co-polymerization with muscle actin. J. Biol. Chem. 251: 7474-7479, 1976.